Age-related changes in soleus ia reflex characteristics during quiet standing


BACKGROUND: Age-related sensorimotor changes are thought to alter the Ia reflex pathway in various ways, including reduced muscle spindle sensitivity, slowed nerve conduction velocity, and reduced synaptic transmission. In the Soleus muscle, these changes may affect older adults' control of standing balance, however, previous research has found inconsistent results in standing posture. The objective of this study was to characterize changes in the Soleus muscle response to Achilles noisy tendon vibration (NTV) during standing across a broad age range. METHODS: Soleus surface EMG and forceplate data were recorded while participants (n = 43, age range 20-82) stood quietly. NTV (10 – 115 Hz) was applied to the Achilles tendon at three different amplitudes (5, 10, 15 m/s2). Coherence, phase, and cross-covariance were estimated between vibration acceleration and rectified surface EMG. Reflex scaling was calculated as the slope of peak-to-peak cross-covariance versus stimulus amplitude. Pearson product moment correlation coefficients were calculated between age and (1) peak-to-peak cross-covariance (2) reflex scaling, and (3) centre of pressure (COP) mean power frequency (MPF). RESULTS: Peak-to-peak cross-covariance between NTV and Soleus EMG was negatively correlated with age (r= -0.68), as was reflex scaling with stimulus amplitude (r = -0.50). COP MPF was positively correlated with age (r = 0.44). CONCLUSIONS: The results suggest that with age, the vibration-evoked Soleus response during standing decreases in strength and amplitude scaling. These findings may be related to the changes in balance control observed with normal aging.

Acknowledgments: Funded by NSERC